Stanford Microfluidics Laboratory

Electrokinetic Instability in Microflows
With Conductivity GradientsMotivation As electrokinetic microfluidic systems increase in complexity, robustness to heterogeneous
sample streams becomes a major concern. One critical factor is conductivity gradients in
the buffer stream, which may occur due to imperfect sample preparation, or occur
intentionally in sample stacking processes. We have identified and are currently studying
an electrokinetic flow instability that occurs in DC-power-driven microflows with high
conductivity gradients and Reynolds numbers (Re) as low as 0.05. Our aim is to provide
design guidelines for complex systems by identifying the critical parameters in this
instability, and to leverage the instability in a rapid field-controlled mixer that can be
readily integrated with on-chip electrophoresis.

Project Description Electrokinetic instability is demonstrated on an in-house built glass T-junction. Borate
buffer of different conductivities are electrically driven from the side channels to the
mixing channel. The electroosmotic flow in the mixing channel is from left to right. The
bottom side channel is seeded with Rhodamine B to aid flow visualization.

The following video shows the convective nature of the electrokinetic instability induced
by conductivity gradients. Upon application of unstable electric field, disturbances
originate from the intersection and grow while being convected downstream.

Figure 1 shows the influence of conductivity gradient on flow stability. Given the same
electric field, the flow with matched conductivity is stable, while the flow with a
conductivity ratio of 10:1 is unstable.

Figure 1. Conductivity gradient induced flow instability

Figure 2 shows the influence of electric field on the amplitude of instability. Given the
same conductivity ratio of 10:1, instability is observed at electric field above the
critical value of 0.45 kV/cm, and the strength of instability grows with growing electric
field.

Figure 2. Electric field dependence of flow instability

Currently, we are investigating the mechanisms behind the observed instability. In
particular, we are developing a linear stability analysis to capture the convective
instability with strong dependence on conductivity ratio and electric field strength.
Please contact us for details.